1. Technical Field
This invention relates to the manufacture of coils for use in magnetic resonance imaging spectroscopy (MRIS).
2. Related Art
Magnetic resonance imaging and spectroscopy (MRIS) systems generally comprise a plurality of cylindrical concentric coils which are located around a region within which a patient can be located. The coils include an outermost DC coil which is used to provide a strong constant magnetic field, an inner radio frequency (RF) coil arrangement which is arranged concentrically within the DC coil and a gradient coil assembly which is located between the RF coil and the outer DC coil. The gradient coil assembly is arranged to generate a time-varying audio frequency magnetic field which causes the response frequency of the nuclei of the patient to depend upon their positions within the field. The coils which generate the strong constant magnetic field are generally super-conducting coils. The presence of a patient in the magnetic field may distort the main magnetic field making it insufficiently uniform for imaging or spectroscopic measurements. A known way of counter-acting this effect is by providing multi-turn electrical windings known as shim coils and driving DC electrical currents through those windings. A typical high performance MRIS system may contain 8 to 12 shim coils, each of which is arranged to correct an inhomogeneity with a particular spatial form. The shim coils can also be used to correct intrinsic inhomogeneities of the super-conductive magnet itself.
It is common practice to incorporate shim coils within the structure of the actively shielded gradient coil assemblies which are switched rapidly on and off in a precisely timed sequence to generate MR images. The gradient sequence contains a range of frequencies from zero to 10 kHz or more and this is often referred to as “audio frequency”.
Magnets with different geometries are currently being developed and these include what are known as open magnets with bi-planar gradient and shim assemblies. The present invention is applicable equally to these new geometries.
Shim coils can be divided into two classes. The first class is made up of axial shim coils which comprise a series of complete circular turns and which generate axisymmetric field components. The second class is known as transverse shim coils and these include multiple saddle coils which are typically disposed symmetrically on the surface of a cylinder or some other surface. Depending upon the field component to be corrected a transverse shim can typically comprise 2, 4 6, 8 or 12 saddles connected in series with successive saddles having alternating signs as shown for example in FIG. 2 of the drawings.
A number of methods are known for fabricating shim coils. These include photo-etching and winding using insulated conductor. The present invention is concerned with an improved method for fabricating shim coils which is applicable not only to saddle type arrangements, but also to axial shim coils.